The present invention relates to an optical head which is employable in an optical data recording/ reproducing device for recording or erasing data on or from the data surface of the optical disk by converging a laser beam radiated from a laser beam source to the data surface of an optical disk through an optical system, and reading recorded data by detecting a laser beam reflected at the data surface.
A conventional system of an optical head to be provided within an optical disk type data recording/reproducing device and the like has been arranged as shown in FIG. 1.
In this type of optical head, a laser beam radiated from a semiconductor laser 1 is made in to a parallel luminous flux by a collimator lens 2, and passes through a beam splitter 4 as a beam splitting element with which a prism 3 for converting an elliptic shape in a cross section of the laser beam into a circular shape is integrally formed, further reflected by an erecting mirror 5 to be directed in the direction of an optical disk 10 as well as converged on and projected to the data signal recording surface on the optical disk 10 by an objective lens 6.
Data is recorded by physically or chemically changing the data signal recording surface by a laser beam energy projected onto the surface, and data is read or reproduced in such a manner that a laser beam, which is reflected at the data signal recording surface and reversely travels a path along which the laser beam has previously traveled, is reflected at a right angle by the half mirror surface 4a of the beam splitter 4 and reaches a photo detector (not shown) after passing through a phase plate, i.e., (1/2).lambda. plate 9, so that the laser beam is detected by the photo detector to provide a reproducing signal. Note that a tracking error signal and focusing error signal are obtained from the photo detector in addition to the reproducing signal, and an operation of the optical head is controlled so as to be located at the desired position based on these error signals.
With the above-described arrangement of such an optical head, the beam splitter 4 having a transmittance of 60 through 80%, and thus when the laser beam radiated from the semiconductor laser 1 passes through the beam splitter 4, a part, i.e., 20 through 40%, thereof is reflected at the half mirror surface 4a and directed to the outside of the beam splitter 4. The laser beam having been reflected and directed to the outside of the beam splitter 4 does not contribute to record and reproduce data, but it is converged on a monitoring photo detector 8 by a monitoring converging lens 7 and an output signal corresponding to the converged laser beam from the monitoring photo detector 8 is fed back so as to control an operation of the semiconductor laser 1 to thereby provide a stable laser beam output.
More specifically, an amount of the luminous flux directed to the outside of the beam splitter 4 is proportional to an amount of a laser beam radiated from the semiconductor laser 1, and thus the laser beam output of the semiconductor laser 1 can be controlled so as to be made stable in accordance with an amount of the luminous flux detected by the monitoring photo detector 8.
Nevertheless, with the above-described arrangement, a problem arises in that the optical head is made large as a whole, because, as shown in FIG. 2 which is a cross sectional view taken along the line I--I of FIG. 1, the monitoring converging lens 7 is disposed in the vicinity of the beam splitter 4 and the monitoring photo detector 8 must be disposed at the focusing position of the converging lens, and these parts for monitoring must be arranged not to interfere with an actuator and the like of the objective lens. Further, it is necessary to combine a multiplicity of constituting parts with high accuracy, thus, production cost becomes expensive.